Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$37.49	6.541%	$39.9422	$39.94	$39.95	$39.90
Purchase #2	$100.89	7.777%	$108.7362	$108.74	$108.75	$108.70
Purchase #3	$155.89	11.418%	$173.6895	$173.69	$173.70	$173.70
Purchase #4	$249.83	10.129%	$275.1353	$275.14	$275.15	$275.10
Purchase #5	$168.99	9.274%	$184.6621	$184.66	$184.65	$184.70
Purchase #6	$35.89	13.240%	$40.6418	$40.64	$40.65	$40.60
Purchase #7	$94.74	9.659%	$103.8909	$103.89	$103.90	$103.90
7 purchase totals =	$843.72		$926.6980	$926.70	$926.75	$926.60